finding max value in a row and reporting colum name

Hi,

Hopefully someone can point me in the right direction on how I would go
about solving the following.

I have some data and need to find the column name of the maximum value in
each row.

This could be the data...

a <- data.frame(x = rnorm(4), y = rnorm(4), z = rnorm(4)) 
a

           x           y          z
1  1.6534561  0.11523404  0.2261730
2 -1.2274320 -0.24096054  1.5096028
3 -1.4503096  0.07227427  1.6740867
4  0.1867416  1.25318913 -0.7350560

Here is what I need to generate...

1 x
2 z
3 z
4 y

Any pointers would be appreciated.

Regards,